141. Linked List Cycle 判断链表是否有环

class Solution {
public:
    bool hasCycle(ListNode *head) {
        if(head == NULL)
            return false;
        if(head->next == NULL)
            return false;
        ListNode* p1 = head;
        ListNode* p2 = head;
        p1 = p1->next;
        p2 = p2->next->next;
        while(p2 != NULL && p2->next != NULL && p2->next->next != NULL && p1 != p2){
            p1 = p1->next;
            p2 = p2->next->next;
        }
        if(p1 == p2)
            return true;
        else
            return false;
    }
};